1.4 Sea-Floor Spreading

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Vocabulary

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mid-ocean ridge: the largest chain of mountains in the world
sonar: a device that bounces sound waves off under water objects and then records the echoes of the waves to determine how far the object is
sea-floor spreading: the process that continually adds new material to the ocean-floor
deep-ocean trenches: forms where the oceanic crust bends downwards
subduction: the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle

Outline

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Introduction

• strange enviorment provides the best evidence for continental drift

Mapping the Mid-Ocean Ridge

• they use sonar to map the ocean
  • Iceland is part of the mid-ocean ridge
  • twice as deep as grand canyon
  • scientists curious about how the mid-ocean ridge got there

Evidence for Sea-Floor Spreading

• Sea-Floor Spreading
  • ocean floors carry continents with them
  • molten material rises from the mantle and erupts
  • molten material spreads out, pushing older rock to both sides of a ridge

Molten Material

• molten material erupts again and again

Magnetic Stripes

• patterns of rock show more evidence
  • 780,000 years ago magnetic poles reversed
    • holds a record of the earth's magnetic field

Drilling Samples

• They drill holes in the ocean floorro determine how old the rock is
  • rocks closer ro ridges are younger
  • rocks farther from a ridge are older
    • shows that sea floor spreading has really taken place

Subduction at Deep Ocean Trenches

• can change size and shape of oceans
  • the subduction at the Pacific Ocean is making it shrink
  • the subduction at the Atlantic Ocean is making it expand
    • as the Atlantic expands, the continents move with it

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